In recent years, plastic products have been gradually replacing glass or metal products in view of processability, weight reduction and others. However, plastic products have a problem of being susceptible to surface damage. In order to solve such a problem, methods have been proposed in which a plastic surface is covered with a film having a high surface hardness.
Patent Literature 1 proposes formation of a film of a (meth)acrylate ester copolymer on a surface of a polycarbonate product and succeeding formation thereon of a hardened film of a ladder silicone oligomer containing a silicone surfactant. Also, Patent Literature 2 proposes coating a hardened film of a silicone oligomer containing a colloidal silica onto a plastic substrate. However, these methods have problems of resulting in the failure to provide sufficient adhesion to a plastic substrate and sufficient surface hardness.
A silicon dioxide or other inorganic film can be formed on a plastic substrate such as by a method wherein a sol of silicon dioxide is applied onto the plastic substrate. This method however results in the insufficient adhesion. Also, the use of the plastic substrate prevents the applied film from being heated to a high temperature. This raises a problem of inability to form a film which is high in surface hardness or excellent in gas barrier property. There has been a further problem that the use of vaccum deposition or sputtering process results in the formation of a silicon dioxide or other inorganic film that is very brittle and insufficient in flexibility and adhesion, and also requires a special apparatus.
Also, application of an aqueous suspension of mont-morillonite onto a PET (polyethylene terephthalate) substrate is known to result in the formation of an inorganic film of montmorillonite on the PET substrate. However, the inorganic film such formed shows poor adhesion and insufficient surface hardness and gas barrier property. Also, the montmorillonite film can only be formed onto the PET substrate and can not be formed onto other resin substrates, which has been a further problem.
In Patent Literature 3, it is disclosed that a thin film is formed as a result of application onto a PET substrate of a suspension of a layered titanic acid obtained by treating a layered titanate with an acid and then allowing a basic compound to act on the resultant to effect swelling of spaces between layers or delamination of the layers. However, no study has been made for resin substrates other than the PET substrate. Also, no detailed discussions have been provided concerning thin film-forming conditions and properties of the thin film formed.
Patent Literatures 4-6 disclose production methods of a layered titanate, as will be described hereinafter. Patent Literatures 7 and 8 disclose production methods of a suspension of a lamellar titanic acid.    Patent Literature 1: Patent Laid-Open No. Hei 3-287634    Patent Literature 2: Patent Laid-Open No. Hei 11-43646    Patent Literature 3: International Publication No. WO 03/016218 pamphlet    Patent Literature 4: Patent Registration No. 2979132    Patent Literature 5: International Publication No. WO 99/11574 pamphlet    Patent Literature 6: Patent Registration No. 3062497    Patent Literature 7: Patent Registration No. 2671949    Patent Literature 8: International Publication No. WO 03/037797 pamphlet